Grammar instuctions;
Lexer
id = ('a'..'z')+;
blank = ' ' | #9 | #10 | #13;

Parser
Ignored blank;

prog = stmt* ;
stmt =
  {assign:} id '=' expr ';' |
  {print:} 'print' '(' expr ')' ';' |
  {while:} 'while' '(' expr ')' '{' stmt* '}'
//alt1 | {until:} 'do' '{' stmt* '}' 'until' '(' expr ')' ';'
//alt2 | {if:} 'if' '('expr')' '{'stmt*'}' elsex?
//alt3 | {call:} id '(' args ')' ';'
  ;

//alt2 elsex = 'else' '{' stmt* '}' ;
//alt3 args = (expr Separator ',')* ;

expr = id;